1. Field of the Invention
The present invention relates to a cylinder liner and a cylinder block having a cast cylinder liner therein to be used for an engine.
2. Discussion of the Related Art
A widely-used cylinder block for an engine is made of an aluminum alloy for decreasing the weight thereof and achieving low fuel consumption. For producing an engine having a good abrasive resistance, a cast iron cylinder liner is provided on the inner surface of a cylinder block main body.
However, it is possible that, in the production by a conventional cylinder block having a cylinder liner, gaps or voids are formed at the interface between the cylinder block main body and the cylinder liner.
When a gap is formed at the interface between the cylinder block main body and the cylinder liner, the thermal conductivity therebetween is decreased. Accordingly, the cooling process of the engine can be influenced, and the thermal conductivity in the cylinder liner varies depending on the circumferential position of the cylinder liner. The variation of the thermal conductivity of the cylinder liner causes the thermal expansion ratio of the cylinder liner to vary depending on the circumferential position thereof.
Because of the above, it is possible that the cylinder liner expands without maintaining a perfect circular shape, and the inner surface of the cylinder liner, i.e., inner surface of the cylinder bore is deformed to have a distorted cylindrical shape. A piston reciprocatingly moves in the deformed cylinder bore, so that the coefficient of friction between the piston and the cylinder liner is increased. As a result, engine oil consumption and abrasion of the piston ring are increased, and hence this can be a cause of increased fuel consumption, decrease of performance, and short life of the engine.
Furthermore, it is possible that water may penetrate into the gap formed at the interface between the cylinder liner and the cylinder block main body. In this case, the cylinder liner can corrode, and the corrosion may lead to deformation of the cylinder liner.
A load is applied to the cylinder liner in the course of treating/processing the inner surface of the cylinder bore. When the gap is formed at the interface between the cylinder block main body and the cylinder liner, the load is applied non-uniformly to the cylinder liner. Accordingly, elastic deformation, that is, spring-back of the cylinder liner occurs, and a cylinder block is manufactured with decreased accuracy. When a load is repeatedly applied to the cylinder liner, the cylinder liner is deformed with the passage of time.
Likewise, when the cylinder block main body is processed by a machine, a load is applied non-uniformly to the cylinder block around the gap. Then a part of the cylinder block main body with a small thickness, which is formed around the gap, causes elastic deformation when a load is applied thereto. Accordingly, it is difficult to manufacture a cylinder block with good accuracy.
An aluminum cylinder block is formed by casting an aluminum alloy around a cylinder liner. In the course of the solidification of the aluminum alloy, the interface between the cylinder liner and the cylinder block main body receives a large load generated by the residual stress mainly of the aluminum alloy, and the thermal expansion ratio comes to be difference between the aluminum alloy and iron for the cylinder liner. When a gap is formed at the interface between the cylinder liner and the cylinder block main body, the stress is concentrated in the parts around the gap. Therefore, it is possible that an aluminum alloy cylinder block main body is damaged. In particular, a part of the cylinder block main body with a small thickness may be damaged when the stress is concentrated in the part.
As a countermeasure, a method for producing a cylinder block is known. Namely, a shot blasting is carried out with respect to the outer surface of the iron cylinder liner by using steel in the form of particles, for activating the surface and for obtaining a rough surface. When an aluminum cylinder block is manufactured with the cylinder block, a close contact is obtained at the interface between the cylinder liner and the cylinder block main body.
In addition to the above, other processes for preparing cylinder blocks are disclosed in Japanese Kokai Publications 2001-227404, 2001-334357, and 7(1995)-139419. According to the publications, a plurality of grooves or protrusions in the form of stripes is integrally formed in the surface of the cast iron cylinder liner. The cylinder liner and the cast cylinder block main body are closely contacted with each other.
Furthermore, another method for producing a cylinder block is known. In the method, a metal is applied to the cylinder block by plating. Examples of the metal in the method include a Cu-based metal and Zn-based metal which having good fusing characteristics with respect to the melt of the aluminum alloy. Then, a gas component such as hydrogen contained in the plated layer is eliminated by immersing the cylinder liner in a flux bath. Subsequently, the thus treated cylinder liner is provided in the cylinder block main body by casting aluminum therebetween. Accordingly, a close contact is obtained at the interface between the cylinder liner and the cylinder block main body.
The above-mentioned method by use of the shot blasting can be carried out by the expense of relatively small cost, and the flowability of the aluminum alloy is increased. Moreover, the contact between the cylinder block main body and the cylinder liner is increased. On the other hand, the bond strength between the cylinder block main body and the cylinder liner is low. Therefore, the cylinder liner tends to be affected by stress such as residual stress or shrinkage generated by the solidification of the melt of the aluminum alloy used for casting. Consequently, it is difficult to obtain a regularly formed interface between the cylinder block main body and the cylinder liner.
In the method disclosed in the previously mentioned publications, where a plurality of grooves or protrusions in the form of stripes is integrally formed on the outer surface of the cylinder liner, the bonding strength is increased to some extent by a mechanical reason. On the other hand, however, the grooves or the protrusions in the form of stripes hinder the flow of the melt of the aluminum alloy. Therefore, it is possible that the interface between the cylinder liner and the cylinder block main body has an irregular contacting state. In other words, close contacting state is partially obtained at the interface. Moreover, there are limitations for forming a plurality of protrusion on the outer surface of the cylinder liner by the treatment by a machine, and it is possible that the manufacturing cost is increased.
In the above-mentioned technology wherein a metal such as Cu-based or Zn-based metal is plated on the outer surface of the cylinder liner, the thickness of the layer obtained by plating (plating layer) with Cu-based material or Zn-based material could be varied. Therefore, the contacting state between the cylinder liner and the plated layer may be made irregular. Such variation and irregularity largely influence the surface structure of the cylinder liner. If the thickness of the plating layer, or contacting state between the plating layer and the cylinder liner varies when the melt of aluminum alloy is introduced, a metal compound is formed by the reaction between the plating layer and the aluminum alloy. As a result, a layer with non-uniform thickness is obtained from the metal compound. Consequently, irregular interfaces are formed, and the interface may have a gap and unstable bonding strength.